In such driving tools, the primary circuit is connected to an AC voltage supply, and contains in series the solenoid and a semiconductor switch that can be turned on upon the application of a trigger signal by a trigger control circuit for an adjustable part of a half-cycle of the AC voltage, whereby the trigger control circuit is activated upon the closing of an associated trigger switch. Electromagnetic-tackers of this kind are well known and are often equipped with electronic circuits for controlling the intensity of the blow such as is suggested in U.S. Pat. No. 3,267,337.
The electronic control circuit disclosed in the above-mentioned U.S. Patent includes a thyristor which, upon every driving impact, controls the power supply to the solenoid each time during a half-cycle of the supply voltage. Because the trigger switch is located in the electronic control circuit, the release, that is the opening, of the trigger switch does not prevent current flowing through the solenoid. Consequently, if the thyristor should fail and short-circuit, the solenoid remains connected to the power supply and can continuously conduct current. In such a case, the solenoid generates excessive heat and under certain circumstances is eventually destroyed. Also, as the supply voltage is not isolated by the trigger switch and is continuously applied, there is the risk of accidental firing of the thyristor due to radio frequency interference generated by universal motors, neon tubes, etc. As a result, the operator of the tacker and other persons working in the immediate vicinity are endangered.
A switch in the main circuit of an electromagnet in series with a semi-conductor switch in the form of thyristor or triac is disclosed in British Pat. Nos. 1,383,086 and 1,476,102. However, in these circuits voltage is applied to the solenoid not only during a part of a half-cycle of the supply voltage, but also continues to be applied until the armature has been pulled up. This is then followed by switching to a weaker holding current which flows either via an additional coil or in a phase cut via the main coil. The switch in the main circuit remains closed after the armature has been pulled up so that the main switch does not protect the solenoid coil in the event of a failure of the thyristor or triac.